A combined cycle power plant is understood here as meaning a combined cycle gas and steam power plant or a gas and steam turbine power plant, which combines the principles of a gas turbine power plant and a steam turbine power plant. A gas turbine serves in this case as a heat source for a downstream boiler, which in turn acts as a steam generator for the steam turbine.
With this combination, a greater efficiency is achieved in the thermodynamic cycle than with gas turbines in open operation or in conventionally fired steam power plants. With electrical efficiencies of up to 60%, combined power plants are among the most efficient conventional power plants.
Furthermore, such combined power plants can be used very flexibly in power plant management. On account of short start-up times and the possibility of quick load changes, they are ideal mid-load power plants. These power plants are primarily operated in the mid-load range and, if need be, even in the range of peak current.
However, below a minimum power output, the gas turbine does not have a constant exhaust-gas temperature. When shutting down the gas turbine, this also leads to a falling of the steam temperature. If the steam turbine then continues to be operated, this leads to a high lifetime consumption or shortening of the lifetime.
Therefore, in conventional combined cycle power plants, the steam turbine is shut down before the gas turbine is brought into the lower load range. For the time period of the steam turbine shutting-down procedure, the gas turbine must stay at a constant power level. This leads to a comparatively long shutting-down process—in comparison with starting up. During shutting down, the efficiency is often not optimal.
EP 2 775 106 A1 discloses a method for operating a combined cycle power plant with a gas turbine and a steam turbine for operating with a minimal load. With respect to shutting down, EP 2 775 106 A1 proposes that a de-loading of the steam turbine and a reducing of the steam pressure are chosen in such a way that a minimal steam turbine load and a minimal steam pressure are reached before a reducing of the steam temperature, caused by a reducing of the gas turbine exhaust-gas temperature during the de-loading of the gas turbine.
EP 2 664 749 A1 also discloses a combined cycle power plant. A controller is provided, producing a flow of steam through a heat recovery device after shutting down.
EP 1 275 817 A1 also discloses a combined cycle power plant that has, along with a gas turbine and a steam turbine, an emergency shut-off oil system, which can stop just the steam turbine or alternatively, in the event of a technical defect, also the gas turbine.
There is therefore a need at least to demonstrate a way in which the shutting down can be sped up.